If this is possible, what is the process for achieving this?
It's actually quite easy, if you know how to do operations in $GF(2^{128})$, specifically, addition, multiplication and computing inverses (note that you already need to know how to do addition and multiplication to perform GHASH in the forward direction).
What GHASH does is take the message [1] and convert it into a series of 128 bit values $a_n, a_{n-1}, ..., a_1$; and it does it in such a way that the bits from $a_n, a_{n-2}, ..., a_2$ come directly from the message. Then, it computes:
$$GHASH_H(\text{Message}) = a_nH^N + a_{n-1}H^{N-1} + ... + a_1H^1$$
So, to find a message that GHASH's to a specific value $T$, you select a message length and a position 16 bytes long which set a specific $a_j$ value (for any $j$). Then, set the bytes on that position to 0 for now, and set the rest of the message bytes arbitrarily, and evaluate the GHASH for that message, it will be a value $T'$.
Then, set those 16 bytes to the value that makes $a_j = H^{-j}(T + T')$, and you're done; when you GHASH that modified message, it'll evaluate to $T$.
[1]: As defined in GCM, GHASH actually takes two messages; one the AAD and one the ciphertext. This doesn't complicate things, and so I'll ignore it
